The present invention relates to vessels for high pressure gas used in space engines, launchers, satellites, probes, . . .
Space launchers have vessels containing products adapted to be pressurized or emptied, such as fuels of the fusing type. To do this, there are used vessels of high pressure gas which permit carrying out the pressurization or emptying. This avoids mechanical systems and such arrangements permit very rapid actions, very precise and requiring control means based on electrovalves, which is less consumptive of energy and involves minimum added weight.
The vector gas under pressure should have particular characteristic such as light weight, absence of risk of combustion. Because of this, helium is preferably used, particularly compared to hydrogen as to the absence of risk of explosion or fire.
Nevertheless, for certain applications, nitrogen, oxygen or rare gases, neon, xenon can be envisaged.
These vessels have a shape which is spherical, cylindrical-spherical, cylindrical-elliptical or torric.
Another interest of these vessels under high pressure is to permit the use of detectors to determine the volume of product contained in the principal vessel, for example the quantity of fusing fuel in the case of the combustible.
Thus, these vessels have a portion of their contents consumed during launching for example and then the remaining portion serves for the other maneuvers and the supply of other motors. Also, it is very useful to determine the remaining quantity and hence the duration of the remaining life, this in an easy fashion starting with temperature and pressure measurements.
French patent application FR-A-2 730 831 discloses such an arrangement in the general framework of a device for pressurizing a subsystem for the propulsion of a geostationary satellite.
The production of vessels fulfilling these requirements is not simple and at present, the vessels are entirely metallic, or of composite material with an internal metallic skin which serves as a core and an external filamentary winding of resin-impregnated fibers.
The advantage of the latter embodiment is weight saving for identical mechanical performance.
An example is given in U.S. Pat. No. 5,822,838, which discloses a process for the production of vessels in titanium alloy with a composite graphite/exopy covering.
In the case of these composite vessels, the internal skin is of metal and performs several functions.
It constitutes an envelope sealed to gases and particularly to helium whose molecules are known to be of very small diameter and which escape most materials with a high rate of loss. Only certain metal alloys are proof against such high diffusion.
The internal skin also serves as a core for winding and absorbs the mechanical stresses during these operations of depositing fibers and pulmerization of the resins.
The internal skin is also used as an anchoring point for the securement of the final vessel so as to ensure the mounting on the structure of the launcher.
On the other hand, such vessels have numerous drawbacks because their production is very sensitive.
Thus, the process of production consists in starting with a rough blank of considerable thickness which is forged, machining to reduce its thickness, which requires a very long time for machining, a high consumption of precious material of which a large part ends as machining scrap.
The parts are then assembled by equatorial welding by electron bombardment. Such an equatorial welding by electronic bombarding is difficult and costly.
Also, a metallic framework of this type constitutes substantially half the cost of the final vessel.
On the other hand, changing material is not simple. When searching among other metals or metallic alloys, one is always faced with a production process which remains complicated because there are hardly alternatives to forging and machining, at least for large vessels of the order of one meter in diameter. Moreover, there can arise problems of compatibility with the gases, which leads to producers selecting titanium as the material, although it is of high cost.
An alternative would be to use plastic materials whose cost of use is much less. On the other hand, if the plastic materials are known, it is also known that they are permeable to gases of high diffusivity such as helium and the amount of loss is prohibitive, in particular for operations of long duration such as satellites whose lifetime is often of the order of 10 to 15 years or even more.
Thus, in this case, not only the pressure decreases too much but the loses of gas can impair the environment of the satellite and particularly the onboard apparatus.
The document EP-0 465 252 discloses a vessel comprising a first skin of polymer, ensuring sealing under certain circumstances. However, the vessel thus obtained does not permit the storage of gases of high diffusivity, such as helium, with an acceptable rate of loss for a storage of long duration.